Semiconductor device

ABSTRACT

A semiconductor device according to the present invention includes a semiconductor layer of SiC of a first conductivity type, a plurality of body regions of a second conductivity type formed in the surface portion of the semiconductor layer with each body region forming a unit cell, a source region of the first conductivity type formed in the inner portion of the body region, a gate electrode facing the body region across a gate insulating film, a drain region of the first conductivity type and a collector region of the second conductivity type formed in the rear surface portion of the semiconductor layer such that the drain region and the collector region adjoin each other, and a drift region between the body region and the drain region, wherein the collector region is formed such that the collector region covers a region including at least two unit cells in the x-axis direction along the surface of the semiconductor layer.

TECHNICAL FIELD

The present invention relates to a silicon carbide (SiC) semiconductordevice.

BACKGROUND ART

In recent years, a SiC semiconductor device has drawn attention, whichis mainly used for systems in various fields related to powerelectronics such as a motor control system, a power conversion systemand so forth.

A patent literature 1 discloses a vertical IGBT, which includes ann-type drift layer formed on a SiC substrate, a p-type base regionformed on the upper portion of the drift layer, and an n-type emitterregion formed on the upper portion of the base region.

A patent literature 2 discloses a trench gate MOSFET that includes an n⁺type SiC substrate, an n⁻ type base layer formed on the SiC substrate, ap-type body region formed in the surface portion of the base layer, ann⁺ type source region formed in the surface portion of the body region,a gate trench passing through the source region and the body region fromthe surface of the base layer, and a gate electrode embedded in the gatetrench across a gate insulating film.

PRIOR ART DOCUMENT Patent Literature

Patent literature 1: Japanese Unexamined Patent Application Publication2011-49267

Patent literature 2: Japanese Unexamined Patent Application Publication2011-44688

Patent literature 3: Japanese Unexamined Patent Application Publication2010-251517

Patent literature 4: Japanese Unexamined Patent Application Publication2010-74051

SUMMARY OF THE INVENTION

The present invention provides a SiC semiconductor device excellent inboth the characteristics of a small current region and thecharacteristics of a large current region.

MEANS FOR SOLVING THE PROBLEM

An embodiment according to the present invention provides asemiconductor device, which includes a semiconductor layer of SiC of afirst conductivity type, a plurality of body regions of a secondconductivity type formed in the surface portion of the semiconductorlayer with each body region forming a unit cell, a source region of thefirst conductivity type formed in the inner portion of the body region,a gate electrode facing the body region across a gate insulating film, adrain region of the first conductivity type and a collector region ofthe second conductivity type formed in the rear surface portion of thesemiconductor layer such that the drain region and the collector regionadjoin each other, and a drift region provided between the body regionand the drain region, wherein the collector region is formed such thatthe collector region covers a region including at least two of the unitcells in the x-axis direction along the surface of the semiconductorlayer.

According to this configuration, the semiconductor device has aHybrid-Metal Oxide Semiconductor (Hybrid-MOS) structure in which a SiCMetal Oxide Semiconductor Field Effect Transistor (MOSFET) and a SiCInsulated Gate Bipolar Transistor (IGBT) are integrated in the samesemiconductor layer. More specifically, the SiC-MOSFET is formed of asource region, a drain region, and a gate electrode, and the SiC-IGBT isformed of a source region, a collector region, and a gate electrode. TheSiC-MOSFET and the SiC-IGBT are connected in parallel via thesemiconductor layer. When the semiconductor device operates as aSiC-IGBT, the source region functions as an emitter region, and thedrift region functions as abase region.

A SiC-MOSFET can be effectively used in a low withstand voltage region(for example, 5 kV or less). That is, when a SiC-MOSFET is turned on, adrain current starts to flow from the time when a drain voltage is 0 V,and then linearly increases with an increase in drain voltage.Accordingly, a SiC-MOSFET can exhibit favorable characteristics of asmall current region. Whereas, since a drain current linearly increaseswith an increase in drain voltage, when a SiC-MOSFET is used in a largecurrent region, the area of a semiconductor layer needs to be expandedwith an increase in drain voltage applied.

On the other hand, a SiC-IGBT can be effectively used mainly in a highwithstand voltage region (for example, 10 kV or greater). That is, sincea SiC-IGBT has conductivity modulation characteristics specific to abipolar transistor, a SiC-IGBT is capable of controlling a large currentwith a high withstand voltage. Accordingly, a SiC-IGBT can exhibitfavorable characteristics of a large current region without expandingthe area of a semiconductor layer. Meanwhile, since SiC is a wide gapsemiconductor, SiC has a high pn barrier compared to Si. As such, when aSiC-IGBT is used in a small current region, a parasitic diode is formedat the pn junction portion, and therefore a relatively high thresholdvoltage of pn junction (for example, 2.7 V or greater) is required.

Thereby, with a SiC-MOSFET and a SiC-IGBT integrated in the samesemiconductor layer, theoretically, a wide operation range can beachieved from a low withstand voltage region to a high withstand voltageregion. That is, a semiconductor device can be provided, which can beused as a high withstand voltage element while carrying out a MOSFET(unipolar) operation in a small current region and carrying out an IGBT(bipolar) operation in a large current region.

However, simply joining the collector region and the drain region of theSiC-MOSFET and the SiC-IGBT may possibly increase an on-resistance orrequire an extremely high voltage during the transfer from a smallcurrent region to a large current region.

Here, in a semiconductor layer wherein a SiC-MOSFET and a SiC-IGBT areformed, an equipotential surface is distributed such that electricalpotential increases from the front surface toward the rear surface of asemiconductor layer. Particularly, in the semiconductor layer, theequipotential surface is distributed such that a relatively highequipotential surface spreads concentrically with the drain region as acenter. As such, when a relatively narrow collector region is formed, anequipotential surface with almost no difference from the drain voltageapplied to the drain region is distributed to cover the collectorregion.

In this case, the drain region and the collector region have the sameelectrical potential, and thus a difference in electrical potential thatis greater or equal to the threshold voltage of pn junction (that is,2.7 V) is hardly generated between the collector region and theequipotential surface that covers the collector region, even if thedrain voltage is increased. As such, the drain voltage needs to beincreased until the difference in electrical potential between thecollector region and the equipotential surface becomes greater or equalto the threshold voltage of pn junction. For this purpose, an extremelyhigh voltage becomes necessary during the transfer from a small currentregion to a large current region.

Here, by forming a collector region so as to cover a region thatincludes at two unit cells in the x-axis direction along the surface ofa semiconductor layer, a relatively low equipotential surface can bewidely distributed over the upper end of the collector region. In thiscase, the difference in electrical potential between the collectorregion and the equipotential surface can be set closer to the thresholdvoltage of pn junction, and therefore the pn junction portion (parasiticdiode) can be turned on with a relatively small increase in drainvoltage. As such, since the transfer from a small current region to alarge current region can be carried out with a relatively small drainvoltage, a tradeoff relationship between the characteristics of a smallcurrent region and the characteristics of a large current region can beimproved. As a result, a SiC semiconductor device excellent in both thecharacteristics of a small current region and the characteristics of alarge current region can be provided.

An embodiment according to the present invention provides asemiconductor device, which includes a semiconductor layer of SiC of afirst conductivity type, a body region of a second conductivity typeformed in the surface portion of the semiconductor layer, a sourceregion of the first conductivity type formed in the inner portion of thebody region, a gate electrode facing the body region across a gateinsulating film, a drain region of the first conductivity type and acollector region of the second conductivity type formed in the rearsurface portion of the semiconductor layer such that the drain regionand the collector region adjoin each other, and a drift region betweenthe body region and the drain region, wherein an x-axis width Wc of thecollector region along the surface of the semiconductor layer is made atleast two times greater than a y-axis thickness Td of the drifter regionalong the thickness direction of the semiconductor layer.

Even with this configuration, the same effect as the previouslydescribed effect can be produced.

According to an embodiment of the present invention, the drain regionmay have an x-axis width Wd that is greater than or equal to that of thecollector region.

With this configuration, the characteristics of a small current regioncan be improved while maintaining the characteristics of a large currentregion.

An embodiment according to the present invention provides asemiconductor device, which includes a semiconductor layer of SiC of afirst conductivity type, a body region of a second conductivity typeformed in the surface portion of the semiconductor layer, a sourceregion of the first conductivity type formed in the inner portion of thebody region, a gate electrode facing the body region across a gateinsulating film, a drain region of the first conductivity type and acollector region of the second conductivity type formed in the rearsurface portion of the semiconductor layer such that the drain regionand the collector region adjoin each other, a drift region between thebody region and the drain region, and an insulating layer arrangedbetween the drain region and the collector region, the insulating layerbeing formed deeper than the drain region and the collector region fromthe rear surface of the semiconductor layer in the y-axis directionalong the thickness direction of the semiconductor layer.

According to this configuration, since an insulating layer is formedbetween the drain region and the collector region, a relatively highequipotential surface expanded from the drain region can be blocked bythe insulating layer. Thereby, a relatively high equipotential surfacecan be prevented from being distributed in the collector region, whileat the same time a relatively low equipotential surface can bedistributed in the collector region. In this case, since the differencein potential between the equipotential surface and the collector regioncan be set closer to the threshold voltage of pn junction, the pnjunction portion (parasitic diode) can be turned on with a relativelysmall increase in drain voltage. As such, since the transfer from asmall current region to a large current region can be carried out with arelatively small increase in drain voltage, a tradeoff relationshipbetween the characteristics of a small current region and thecharacteristics of a large current region can be improved. As a result,a SiC semiconductor device excellent in both the characteristics of asmall current region and the characteristics of a large current regioncan be provided.

According to an embodiment of the present invention, the insulatinglayer may include an insulating film or a high resistance layer.

According to an embodiment of the present invention, the insulatinglayer may include an insulating material having a dielectric constantlower than SiC.

According to an embodiment of the present invention, the insulatinglayer may include SiO₂.

An embodiment according to the present invention provides asemiconductor device, which includes a semiconductor layer of SiC of afirst conductivity type, a body region of a second conductivity typeformed in the surface portion of the semiconductor layer, a sourceregion of the first conductivity type formed in the inner portion of thebody region, a gate electrode facing the body region across a gateinsulating film, a drain region of the first conductivity type and acollector region of the second conductivity type formed in the rearsurface portion of the semiconductor layer, and a drift region betweenthe body region and the drain region such that the drain region and thecollector region adjoin each other, wherein the upper end of thecollector region is positioned closer to the surface of thesemiconductor layer than the upper end of the drain region in the y-axisdirection along the thickness direction of the semiconductor layer.

According to this configuration, since the upper end of the collectorregion is positioned closer to the surface of the semiconductor layerthan to the upper end of the drain region, the equipotential surfaceexpanding from the drain region can be prevented from reaching the upperend of the collector region. Thereby, a relatively high equipotentialsurface can be prevented from being distributed in the collector region,while at the same time a relatively low equipotential surface can bedistributed in the collector region. In this case, since the differencein potential between the equipotential surface and the collector regioncan be set closer to the threshold voltage of pn junction, the pnjunction portion (parasitic diode) can be turned on with a relativelysmall increase in drain voltage.

Further, by increasing the thickness from the upper end of the drainregion to the upper end of the collector region, the characteristics ofan on-resistance can be improved compared to a case where the drainregion and the collector region are formed to have the same thickness.

In an embodiment according to the present invention, the rear surface ofa semiconductor layer extends across the interface between the drainregion and the collector region to make both surfaces flush with eachother.

An embodiment according to the present invention may further include afield stop zone that is formed to extend from the drain region to thecollector region in the x-axis direction along the surface of thesemiconductor layer, and is arranged between the drift region, and thedrain region and the collector region.

According to this configuration, a Field Stop (FS) type semiconductordevice can be provided. A Non-Punch Through (NPT) type semiconductordevice is also known in contrast to a FS type semiconductor device. Inthe case of an NPT type semiconductor device, the semiconductor layerneeds to be formed relatively thick to prevent the semiconductor devicefrom being punched through with a depletion layer generated from theinterface between the body region and the drift region, reaching thebottom surface of the semiconductor layer. In contrast, in the case of aFS type semiconductor device, since the extension of the depletion layercan be blocked by a field stop zone, a punch-through can be suppressed.Therefore, a FS type semiconductor device allows for a thinnersemiconductor layer compared to an NPT type semiconductor device.

An embodiment according to the present invention may include aplanar-gate structure in which the gate electrode is arranged on thesemiconductor layer.

An embodiment according to the present invention may include atrench-gate structure in which the gate electrode is embedded in atrench formed in the semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a SiC semiconductor deviceaccording to an embodiment of the present invention.

FIG. 2A is a graph illustrating theoretical characteristics of drainvoltage (collector voltage) vs. drain current (collector current) in theHybrid-MOS structure, which can be derived from each characteristic of aSiC-MOSFET and a SiC-IGBT that are individually manufactured.

FIG. 2B is a graph illustrating actual characteristics of drain voltage(collector voltage) vs. drain current (collector current) in theHybrid-MOS structure.

FIG. 3 is a graph illustrating the actual characteristics of thresholdvoltage of pn junction vs. characteristic on-resistance in theHybrid-MOS structure.

FIG. 4 is a view illustrating the distribution of electrical potentialwhen changing the ratio of the area of the drain region to the collectorregion.

FIG. 5 is a view illustrating the distribution of electrical potentialwhen changing the ratio of the area of the drain region to the collectorregion.

FIG. 6 is a view illustrating the distribution of electrical potentialwhen changing the ratio of the area of the drain region to the collectorregion.

FIG. 7 is a graph illustrating the characteristics of drain voltage(collector voltage) vs. drain current (collector current) when changingthe ratio of the area of the collector region to the drain region.

FIG. 8 is a graph illustrating the characteristics of drain voltage(collector voltage) vs. drain current (collector current) when changingthe width of the drain region.

FIG. 9 is a graph illustrating the characteristics of a large currentregion in FIG. 8.

FIG. 10A is a schematic cross-sectional view of a SiC semiconductordevice according to an embodiment of the present invention.

FIG. 11 is a view illustrating the potential distribution of a SiCsemiconductor device shown in FIG. 10.

FIG. 12 is a graph illustrating threshold voltage of pn junction vs.characteristic on-resistance of a SiC semiconductor device shown in FIG.10.

FIG. 13 is a schematic cross-sectional view of a SiC semiconductordevice according to an embodiment of the present invention.

FIG. 14 is a view illustrating the distribution of electrical potentialof a semiconductor device shown in FIG. 13.

FIG. 15 is a schematic cross-sectional view of a SiC semiconductordevice according to an embodiment of the present invention.

FIG. 16 is a schematic cross-sectional view of a SiC semiconductordevice according to an embodiment of the present invention.

FIG. 17 is a schematic cross-sectional view of a SiC semiconductordevice according to an embodiment of the present invention.

FIG. 18 is a plan view illustrating a planar shape of a collectorregion.

FIG. 19 is a plan view illustrating a planar shape of a collectorregion.

FIG. 20 is a plan view illustrating a planar shape of a collectorregion.

FIG. 21 is a plan view illustrating a layout example of collectorregions and drain regions.

FIG. 22 is a plan view illustrating a layout example of collectorregions and drain regions.

EMBODIMENT FOR PRACTICING THE INVENTION

Hereinafter, an embodiment according to the present invention will bespecifically discussed with reference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view of a SiC semiconductor device1 according to an embodiment of the present invention.

As shown in FIG. 1, a SiC semiconductor device 1 includes a n⁻ type SiCsemiconductor layer 10 with front and rear surfaces. A plurality ofp-type body regions 12 with each forming a unit cell 11 is formed in thesurface portion of the SiC semiconductor layer 10.

The p-type body regions 12 are formed spaced apart from each other inthe surface portion of SiC semiconductor layer 10. A n-type sourceregion 13 and P⁺ type contact region 14 are formed in the inner portionof the p-type body region 12. The n-type source region 13 is formed at aposition away from the peripheral edge of the p-type body region 12. Ap-type channel region 15 is positioned in a region between theperipheral edge of the n-type source region 13 and the peripheral edgeof the p-type body region 12. The P⁺ type contact region 14 ispositioned in the inner portion of the n-type source region 13 and isformed to pass through the n-type source region 13. The P⁺ type contactregion 14 has a higher impurity concentration than the p-type bodyregion 12. Both the n-type source region 13 and the P⁺ type contactregion 14 are formed shallow than the p-type body region 12.

Mutually adjoining n⁺ type drain regions 16 and P⁺ type collectorregions 17, and an n⁺ type field stop (FS) region 18 extending over then⁺ type drain regions 16 and the P⁺ type collector regions 17 are formedin the rear surface portion of the SiC semiconductor layer 10. The rearsurface of the SiC semiconductor layer 10 extends across the interfacebetween the n⁺ type drain regions 16 and the P⁺ type collector regions17 to make both surfaces flush with each other.

The FS region 18 is formed to have a uniform thickness in the x-axisdirection along the surface of the SiC semiconductor layer 10 so as tocome in contact with the upper end of the n⁺ type drain regions 16 andthe upper end of the p⁺ type collector regions 17. A region between theFS region 18 and the unit cells 11 (p-type body region 12) is a n⁻ typedrift region 19. The y-axis thickness Td of the n⁻ type drift region 19in the y-axis direction along the thickness direction of the SiCsemiconductor layer 10 is, for example, 10 μm-100 μm (46 μm in thisembodiment).

The n⁺ type drain region 16 is formed beneath a region between mutuallyadjacent unit cells 11 (p-type body regions 12). The x-axis width Wd ofthe n⁺ type drain region 16 in the x-axis direction along the surface ofthe SiC semiconductor layer 10 is, for example, 10 μm-100 μm. In thisembodiment, the upper end of the n⁺ type drain region 16 is positionedat the same depth as the upper end of the P⁺ type collector regions 17in the y-axis direction along the thickness direction of the SiCsemiconductor layer 10.

The P⁺ type collector regions 17 forms a pn junction portion with theSiC semiconductor layer 10. That is, a parasitic diode is formed in thepn junction portion. The P⁺ type collector regions 17 is formed to havean area greater than the n⁺ type drain region 16 in the x-axis directionalong the surface of the SiC semiconductor layer 10. More specifically,the P⁺ type collector regions 17 is formed to cover a region includingat least two unit cells 11 in the x-axis direction. The x-axis width Wcof the P⁺ type collector regions 17 in the x-axis direction along thesurface of the SiC semiconductor layer 10 is, for example, 50 μm-100 μm.The x-axis width Wc of the P⁺ type collector regions 17 is preferablyestablished so as to satisfy the following expression in relationship tothe y-axis thickness Td of the n⁻ type drift region 19: x-axis widthWc>y-axis thickness Td×2

Such n⁺ type drain region 16 and P⁺ type collector regions 17 are formedby the following method. First, n⁺ type SiC substrate is prepared. Next,the SiC is epitaxially grown with n-type impurities being injectedtherein, and thus the n⁻ type SiC semiconductor layer 10 is formed onthe SiC substrate. Next, a MOS structure including the p-type bodyregion 12, the n-type source region 13, the later-described gateinsulating film 20, gate electrode 21, source electrode 24 and so forthis formed in the SiC semiconductor layer 10, and thereafter the SiCsubstrate is ground until the SiC semiconductor layer 10 is exposed. TheSiC substrate may also be removed by dry etching instead of the grindingof the SiC substrate.

Next, n-type impurities are selectively injected in the rear surfaceportion of the SiC semiconductor layer 10 to thereby form the FS region18.

Next, an ion injection mask is formed on the rear surface side of theSiC semiconductor layer 10, the ion injection mask selectively providedwith openings corresponding to regions where the n⁺ type drain region 16is formed. N-type impurities are injected through the ion injectionmask. After the impurities are injected, the ion injection mask isremoved.

Next, an ion injection mask is formed on the rear surface side of theSiC semiconductor layer 10, the ion injection mask selectively providedwith openings corresponding to regions where the p⁺ type collectorregion 17 is formed. P-type impurities are injected through the ioninjection mask. After the impurities are injected, the ion injectionmask is removed.

Next, laser annealing is selectively applied to the regions where then-type impurities and p-type impurities are injected. Thereby, then-type impurities and p-type impurities are activated so that the n⁺type drain region 16 and p⁺ type collector region 17 are formed.

The SiC semiconductor layer 10 has a higher density than a semiconductorlayer of Si, and thus has characteristics that impurities are hardlydiffused. As such, by adjusting the injection conditions and theannealing conditions for impurities using the characteristics, thethickness of n-type impurities and p-type impurities can be easilyadjusted. Thereby, the n⁺ type drain region 16 and the p⁺ type collectorregion 17 can be accurately formed.

Again, referring to FIG. 1, a plurality of gate electrodes 21 is formedto face the p-type channel region 15 across the gate insulating film 20on the SiC semiconductor layer 10. The gate insulating film 20 mayinclude, for example, a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a hafnium oxide film, an aluminum film, atantalum oxide film and so forth. Further, the gate electrode 21 mayinclude, for example, a polysilicon having lowered resistance withimpurities injected.

Each gate electrode 21 faces a region extending over the SiCsemiconductor layer 10 outside the p-type body region 12, the p-typebody region 12, and the n-type source region 13. Further the gateelectrode 21 includes an overlapped portion protruded from a boundaryline between the n-type source region 13 and the p-type body region 12toward the n-type source region 13. The insulating film 22 is formed onthe SiC semiconductor layer 10 so as to cover the gate electrode 21.

The insulating film 22 has a contact hole 23 formed so as to selectivelyexpose a portion of the n-type source region 13 and the p⁺ type contactregion 14. The source electrode 24 is formed on the insulating film 22.

The source electrode 24 gets into the contact hole 23 from the surfaceof the insulating film 22, and forms an ohmic contact with the n-typesource region 13 and the p⁺ type contact region 14 in the contact hole23. Meanwhile, a drain electrode 25 is formed on the rear surface sideof the SiC semiconductor layer 10. The drain electrode 25 forms an ohmiccontact with the n⁺ type drain region 16 and the p⁺ type collectorregion 17.

According to this configuration, the SiC semiconductor device 1 has ahybrid metal oxide semiconductor (Hybrid-MOS) structure in which a SiCmetal oxide semiconductor field effect transistor (SiC-MOSFET) and a SiCinsulated gate bipolar semiconductor (SiC-IGBT) are integrated in thesame SiC semiconductor layer 10. The Hybrid-MOS structure is aplanar-gate structure.

More specifically, the SiC-MOSFET is formed by the n-type source region13, the n⁺ type drain region 16, and the gate electrode 21, and theSiC-IGBT is formed by the n-type source region 13, the p⁺ type collectorregion 17, and the gate electrode 21. That is, the SiC-MOSFET and theSiC-IGBT are connected in parallel via the SiC semiconductor layer 10.When the SiC semiconductor device 1 functions as the SiC-IGBT, then-type source region 13 (source electrode 24) functions as the n-typeemitter region (emitter electrode), and the n⁻ type drift region 19functions as n⁻ type base region.

Next, in the light of the electrical characteristics in a case where theSiC-MOSFET and the SiC-IGBT are individually manufactured, thetheoretical electrical characteristics and the actual electricalcharacteristics of the Hybrid-MOS structure will be discussed.

FIG. 2A is a view illustrating the theoretical characteristics of drainvoltage (collector voltage) vs. drain current (collector current) in theHybrid-MOS structure, which can be derived from each characteristic of aSiC-MOSFET and a SiC-IGBT that are individually manufactured. The drainvoltage (collector voltage) implies a voltage applied to the drainelectrode 25 with the source electrode 24 as a reference voltage (forexample 0 V).

The SiC-MOSFET is effective as an element used mainly in a low withstandvoltage region (for example, 5 kV or less). In the case of SiC-MOSFET,as can be seen from a straight line A1 shown as a dashed line in FIG.2A, the drain current begins to flow when the drain voltage is 0 V, andlinearly increases with the increase of the drain voltage. As such, theSiC-MOSFET can exhibit the favorable characteristics of a small currentregion. Meanwhile, since the drain current linearly increased with theincrease of the drain voltage, when the SiC-MOSFET is used in a largecurrent region, the area of the SiC semiconductor layer 10 needs to beexpanded with an increase in drain voltage applied.

On the other hand, the SiC-IGBT can be effectively used mainly in a highwithstand voltage region (for example, 10 kV or greater). That is, sincethe SiC-IGBT has conductivity modulation characteristics specific to abipolar transistor, the SiC-IGBT is capable of controlling a largecurrent with a high withstand voltage. As can be seen from a curved lineA2 shown by a dashed line in FIG. 2A, in the case of the SiC-IGBT, whenthe collector voltage exceeds approximately 2.7 V, the collector currentshows precipitous rise characteristics. Therefore, the SiC-IGBT canexhibit the favorable characteristics of a large current region withoutexpanding the area of the SiC semiconductor layer 10.

Meanwhile, since SiC is a wide gap semiconductor, SiC has a higher pnbarrier. As such, when using SiC in a small current region, a higherthreshold voltage of pn junction (approximately 2.7 V) is required. Thatis, to switch on a parasitic diode D (see FIG. 1) formed between the p⁺type collector region 17 and the SiC diode layer 10, the thresholdvoltage of pn junction (approximately 2.7 V) is required.

From these, it can be seen that a wide operation range from a lowwithstand voltage region to a high withstand voltage region can betheoretically created by integrating the SiC-MOSFET and the SiC-IGBT inthe same SiC semiconductor layer. That is, it can be seen that asemiconductor device can be provided, which is capable of being used asa high withstand voltage element while performing a MOSFET (unipolar)operation in a small current region, and performing an IGBT (bipolar)operation in a large current region. The theoretical characteristics arerepresented by a curved line A3 shown as a solid line in FIG. 2A.

Referring to the theoretical curved line A3, it can be seen that theSiC-MOSFET is turned on at 0 through 3 V and thus favorablecharacteristics in a small current region can be achieved. It can alsobe seen that by setting a voltage applied to a pn junction portion to avoltage more than or equal to the voltage required for forming the pnjunction (approximately 3 V or more), the SiC-IGBT is turned on, andfavorable characteristics in a large current region can be achieved.

FIG. 2B is a graph illustrating actual characteristics of drain voltage(collector voltage) vs. drain current (collector current) in theHybrid-MOS structure. The drain voltage (collector voltage) implies avoltage applied to the drain electrode 25 with the source electrode 24as a reference voltage (for example 0 V). In FIG. 2B, the straight lineA1 and the curved line A2 are continuously shown in dashed lines.

A curved line A4 and a curved line A5 shown in solid lines in FIG. 2B,indicate the characteristics of a SiC semiconductor device in which thex-axis width Wd of the n⁺ type drain region 16 and the x-axis width Wcof the p⁺ type collector region 17 are formed to satisfy a relationalexpression: x-axis width Wd+x-axis width Wc<<12 μm.

Referring to the curved line A4, it can be seen that although the curvedline A4 exhibits favorable small current characteristics (favorableon-resistance), an extremely high threshold voltage of pn junction(approximately 19 V) is required for the transfer from a small currentregion to a large current region. On the other hand, referring to thecurved line A5, it can be seen that the threshold voltage of pn junctionrequired for the transfer from a small current region to a large currentregion is improved, however, the small current characteristics aredegraded compared to the curved line A4.

As can be seen from FIG. 2B, the characteristics of the SiCsemiconductor device vary with each x-axis width Wd, Wc of the n⁺ typedrain region 16 and the p⁺ type collector region 17. As such, it can beseen that some ingenuity is required in making the n⁺ type drain region16 and the p⁺ type collector region 17 in order to move these curvedlines A4, A5 closer to the theoretical curved line A3 shown in FIG. 2A.In this regard, the inventors found that the characteristics of a smallcurrent region and the characteristics of a large current region are inthe tradeoff relationship as shown in FIG. 3.

FIG. 3 is a graph illustrating the actual characteristics of thresholdvoltage of pn junction vs. characteristic on-resistance in theHybrid-MOS structure. The characteristic on-resistance is defined as theinclination of drain current-drain voltage curve when the drain voltageis 0 V.

As described in FIG. 2B, the n⁺ type drain region 16 and the p⁺ typecollector region 17 satisfies the relational expression in associationwith each x-axis width Wd, Wc: x-axis width Wd+x-axis width Wc<<12 μm.That is, the x-axis width Wc of the p⁺ type collector region 17 isformed to satisfy the relational expression with respect to the y-axisthickness Td: x-axis width Wc<<2×y-axis thickness Td. A curved line A6shown as a solid line in FIG. 3 illustrates the actual characteristicswhen satisfying the relational expression: x-axis width Wc<<2×y-axisthickness Td.

Referring to the curved line A6, it can be seen that if an extremelygreater ratio of the area of the n⁺ type drain region 16 to the p⁺ typecollector region 17 is set, the characteristics get closer to that ofthe SiC-MOSFET. That is, a favorable characteristic on-resistance(favorable characteristics of a small current region) can be obtained,however, an extremely high voltage is required during the transfer to alarge current region.

It can also be seen that if an extremely greater ratio of the area ofthe p⁺ type collector region 17 to the n⁺ type drain region 16 is set,the characteristics get closer to that of the SiC-IGBT. That is,although a pn junction can be formed with a relatively low drainvoltage, a characteristic on-resistance significantly increases (smallcurrent characteristics decrease).

A point P1 in the curved line A6 corresponds to the curved line A4 inthe graph shown in FIG. 2B; the value of the characteristicon-resistance is approximately 20 mΩ·cm²; and the drain voltagenecessary for the threshold voltage of pn junction is 19 V. Whereas, apoint P2 in the curved line A6 corresponds to the curved line A5 in thegraph shown in FIG. 2B; the drain voltage in the formation of the pnjunction is 5 V; and the value of the characteristic on-resistance isapproximately 40 mΩ·cm².

As can be seen from the curved line A6, the characteristics of a smallcurrent region and the characteristics of a large current region are inthe tradeoff relationship. Even if the ratio of the area of the p⁺ typecollector region 17 to the n⁺ type drain region 16 is changed, thecharacteristics only fluctuate in the direction of the point P1 or inthe direction of the point P2 so that the tradeoff relationship cannotbe substantially improved.

A curved line A7 shown as a dashed line in FIG. 3 exhibits actualcharacteristics in which the x-axis width Wc of the p⁺ type collectorregion 17 satisfies a relational expression: x-axis width Wc>2×y-axisthickness Td. Referring to the curved line A7, it can be seen that thetradeoff relationship is substantially improved compared to the curvedline A6. That is, the curved line A7 is moved closer to a straight lineA8 representing the lower limit (approximately 2.7 V) of the thresholdvoltage of pn junction (SiC-IGBT) and a straight ling A9 representingthe lower limit (approximately 18 mΩ·cm²) of the characteristicon-resistance (SiC-MOSFET) compared to the curved line A6.

It can be seen that if the value of the threshold voltage of pn junctionand the value of the characteristic on-resistance can be moved closeralong this curved line A7 to a point P3 at which the straight line A8and the straight line A9 intersect with each other, the excellentcharacteristics of a small current region and characteristics of a largecurrent region can be achieved.

Focusing on this, the inventors prepared a plurality of SiCsemiconductor devices in which the ratio of the area of the p⁺ typecollector region 17 to the n⁺ type drain region 16 is sequentiallyincreased, and carried out a simulation for each SiC semiconductordevice.

FIGS. 4 through 6 are views illustrating the distribution of electricalpotential when changing the ratio of the area of the p⁺ type collectorregion 17 to the n⁺ type drain region 16. In FIGS. 4 through 6,structures except with major structures are omitted for the purpose ofillustration.

The x-axis width Wc of the p⁺ type collector region 17 in FIG. 4 is 10μm. The x-axis width Wc of the p⁺ type collector region 17 in FIG. 5 is50 μm. The x-axis width Wc of the p⁺ type collector region 17 in FIG. 6is 100 μm. Any of the x-axis widths Wd of the n⁺ type drain region 16 inFIGS. 4 through 6 are 10 μm. Any of the y-axis thickness Td of the ntype drift region 19 are 46 μm.

FIGS. 4 through 6 show the distribution of potential when a voltage ofapproximately 3 V is applied to the drain electrode 25 with the sourceelectrode 24 as a reference (=0 V). As shown in FIGS. 4 through 6, theequipotential surface is distributed to gradually increase from thesource electrode 24 toward the drain electrode 25. Further, theequipotential surface that has a relatively high potential isconcentrically distributed with the n⁺ type drain region 16 as thecenter.

According to FIG. 4, an equipotential surface with approximately 2.5 Vis distributed over the upper end of the p⁺ type collector region 17.Whereas, in FIG. 5, an equipotential surface with approximately 0.5 Vthrough 2 V is widely distributed over the upper end of the p⁺ typecollector region 17. Therefore, it can be seen that the potentialdifference between the p⁺ type collector region 17 and the equipotentialsurface is greater. Further according to FIG. 6, an equipotentialsurface with approximately 0.5 V through 2 V is further widelydistributed over the upper end of the p⁺ type collector region 17.Therefore, it can be seen that the potential difference between the p⁺type collector region 17 and the equipotential surface is furthergreater compared to FIG. 4 and FIG. 5.

FIG. 7 is a graph reflecting the results of these simulations. FIG. 7 isa graph illustrating the characteristics of drain voltage (collectorvoltage) vs. drain current (collector current) when changing the ratioof the area of the p⁺ type collector region 17 to the n⁺ type drainregion 16.

A curved line L1 exhibits characteristics when the x-axis width Wc ofthe p⁺ type collector region 17 is 10 μm (see FIG. 4); a curved line L2exhibits characteristics when the x-axis width Wc of the p⁺ typecollector region 17 is 20 μm; a curved line L3 exhibits characteristicswhen the x-axis width Wc of the p⁺ type collector region 17 is 50 μm(see FIG. 5); and a curved line L4 exhibits characteristics when thex-axis width Wc of the p⁺ type collector region 17 is 100 μm (see FIG.6).

According to the graph shown in FIG. 7, the drain voltage required forthe transfer to a large current region decreases as the x-axis width Wcof the p⁺ type collector region 17 increases.

According to the structure shown in FIG. 4, there is almost nodifference between the equipotential surface distributed over the upperend of the p⁺ type collector region 17, and the drain voltage appliedonto the n⁺ type drain region 16. With this configuration, the n⁺ typedrain region 16 and the p⁺ type collector region 17 have the samepotential, and thus even if the drain voltage is increased, a potentialdifference greater than or equal to the threshold voltage of pn junction(that is 2.7 V) is hardly generated between the p⁺ type collector region17 and the equipotential surface.

As such, the drain voltage needs to be increased until the potentialdifference between the p⁺ type collector region 17 and the equipotentialsurface becomes greater than or equal to the threshold voltage of pnjunction, thereby requiring an extremely high voltage in the transferfrom a small current region to a large current region. As shown in FIG.7, it can be seen that the potential difference between the p⁺ typecollector region 17 and the equipotential surface covering the p⁺ typecollector region 17 does not become greater than or equal to thethreshold voltage of pn junction unless the drain voltage increases upto approximately 19 V in the structure depicted in FIG. 4.

In contrast, according to the structure shown in FIG. 6, a relativelylow equipotential surface can be distributed over the upper end of thep⁺ type collector region 17. With this structure, the potentialdifference between the p⁺ type collector region 17 and the equipotentialsurface can be made to approach the threshold voltage of pn junction,and thus the pn junction portion (parasitic diode D) can be turned on bya relatively small drain voltage (3.0 through 3.5 V). Thereby, thetransfer from a small current region to a large current region issmoothly carried out.

The graphs shown in FIG. 8 and FIG. 9 exhibit the characteristics ofdrain voltage (collector voltage) vs. drain current (collector current)when changing the x-axis width Wd of the n⁺ type drain region 16 from 10μm to 100 μm with the x-axis width Wc of the p⁺ type collector region 17maintained at 100 μm.

FIG. 8 is a graph illustrating the characteristics of drain voltage(collector voltage) vs. drain current (collector current) when changingthe x-axis width Wd of the n⁺ type drain region 16. FIG. 9 is a graphillustrating the characteristics of a large current region in FIG. 8.

A curved line L5 shown as a solid line in each graph of FIGS. 8 and 9exhibits the characteristics where the x-axis width Wc of the p⁺ typecollector region 17 is 100 μm, and the x-axis width Wd of the n⁺ typedrain region 16 is 100 μm. In each graph shown in FIGS. 8 and 9, thecurved lines L1, L4 shown in the graph of FIG. 7 are shown in dashedlines.

Referring to the graph shown in FIG. 8, it was found that a draincurrent (collector current) in a small current region increased byincreasing the x-axis width Wd of the n⁺ type drain region 16 andthereby relatively decreasing the ratio of the x-axis width Wc of the p⁺type collector region 17. In other words, it was found that the draincurrent (collector current) in a small current region increased due to adecrease in the value of characteristic on-resistance by relativelyincreasing the ratio of the x-axis width Wd of the n⁺ type drain region16. In addition, it was found as shown in FIG. 9 that thecharacteristics in a large current region stayed pretty much the same.

As a result, it was found that the characteristics in a small currentregion could be improved while maintaining a suitable threshold voltageof pn junction (3.0V through 3.5 V) by relatively decreasing the ratioof the x-axis width Wc of the p⁺ type collector region 17. That is, itcan be seen that the characteristics in a small current region can beimproved by forming the x-axis width Wd of the n⁺ type drain region 16to have a width greater than or equal to the x-axis width Wc of the p⁺type collector region 17.

As described above, according to the SiC semiconductor device 1, byforming the p⁺ type collector region 17 to cover a region including atleast two unit cells 11 in the x-axis direction along the surface of theSiC semiconductor layer 10, a relatively low equipotential surface canbe widely distributed over the upper end of the p⁺ type collector region17.

From another point of view, the x-axis width Wc of the p⁺ type collectorregion 17 along the surface of the SiC semiconductor layer 10 is made atleast two times greater than the y-axis thickness Td of the n⁻ typedrift region 19 along the thickness direction of the SiC semiconductorlayer 10, and thereby a relatively low equipotential layer can be widelydistributed over the upper end of the p⁺ type collector region 17.

With this configuration, the potential difference between the p⁺ typecollector region 17 and the equipotential surface can be made toapproach the threshold voltage of pn junction, and thus the pn junctionportion (parasitic diode D) can be turned on with a relatively smallincrease in drain voltage, as shown in FIG. 7. In this way, the transferfrom a small current region to a large current region can be carried outwith a relatively small drain voltage, and thereby the tradeoffrelationship between the characteristics of a small current region andthe characteristics of a large current region can be improved. As aresult, the SiC semiconductor device 1 excellent in both thecharacteristics of a small current region and the characteristics of alarge current region can be provided.

Further, as shown in FIGS. 8 and 9, by forming the n⁺ type drain region16 to have the x-axis width Wd that is greater than or equal to thex-axis width Wc of the p⁺ type collector region 17, the characteristicsof a small current region can be improved while maintaining thecharacteristics of a large current region.

Further, as shown in FIG. 1, the FS region 18 is formed to extend overthe n⁺ type drain region 16 and the p⁺ type collector region 17.Thereby, a field stop (FS) type SiC semiconductor 1 can be provided.Anon-punch through (NPT) type SiC semiconductor device, which does nothave the FS region 18, is known as opposed to the FS type Semiconductordevice 1.

In the case of NPT type SiC semiconductor device, the SiC semiconductorlayer 10 needs to be formed to have a relatively large thickness, whichprevents a depletion layer generated from the interface between thep-type body region 12 and the n⁻ type drift region 19 from reaching thebottom surface of the SiC semiconductor layer 10 thereby punchingthrough the device. In contrast, in the case of the FS type SiCsemiconductor 1, the expansion of a depletion layer can be stopped bythe FS region 18, the occurrence of punch-through can be suppressed.Therefore, according to the FS type SiC semiconductor device 1, thethickness of the semiconductor layer 10 can be reduced compared to anNPT type SiC semiconductor device.

FIG. 10 is a schematic cross-sectional view of a SiC semiconductordevice 2 according to an embodiment of the present invention.

The SiC semiconductor device 2 is different from the previouslydescribed SiC semiconductor device 1 in that a p⁺ type collector region31 is formed in place of the p⁺ type collector region 17. Othercomponents are the same as those shown in the SiC semiconductor device1. In FIG. 10, the same reference numerals are applied to the componentscorresponding to each component shown in FIG. 1 previously described andthe descriptions for such components will be omitted.

As shown in FIG. 10, a plurality of p⁺ type collector regions 31 and aplurality of n⁺ type drain regions 16 are formed to adjoin each other inthe rear surface portion of the SiC semiconductor layer 10. Each p⁺ typecollector region 31 is formed right below each unit cell 11 (p-type bodyregion 12), and faces the unit cell 11 across the n⁻ type drift region19. The x-axis width Wc of the p⁺ type collector region 31 in the x-axisdirection along the surface of the SiC semiconductor layer 10 is, forexample 10 μm.

FIG. 10 shows an example where the x-axis width Wc of the p⁺ typecollector region 31 is made narrower than the width of the p-type bodyregion 12. The x-axis width Wc of the p⁺ type collector region 31 (theratio of the area of the p⁺ type collector region 31 to the area of then⁺ type drain region 16) can be changed in accordance with the y-axisthickness Td of the n⁻ type drift region 19 as necessary. Therefore,when the y-axis thickness Td of the n⁻ type drift region 19 is madethicker, the p⁺ type collector region 31 may be made wider than thewidth of the p-type body region 12.

The upper end of the p⁺ type collector region 31 is positioned closer tothe surface of the SiC semiconductor layer 10 than the upper end of then⁺ type drain region 16 in the y-axis direction along the thicknessdirection of the SiC semiconductor layer 10. A y-axis thickness Dp fromthe upper end of the n⁺ type drain region 16 to the upper end of the p⁺type collector region 31 in the y-axis direction is, for example, 0 μmthrough 10 μm (0 μm<y-axis thickness Dp≦10 μm). The FS region 18according to this embodiment is formed along the upper end and thelateral part of the p⁺ type collector region 31, and the upper end ofthe n⁺ type drain region 16.

Such a p⁺ type collector region 31 can be formed in the same manner asthe previously described manner. That is, the p⁺ type collector region31 can be formed by adjusting injection conditions when injecting p-typeimpurities (for example, doping energy, dose amount and so forth), andannealing conditions (for example, annealing temperature, time and soforth).

FIG. 11 shows the results of researching the potential distribution ofthe SiC semiconductor device 2 in the same manner as describedpreviously in FIGS. 4 through 6. FIG. 11 is a view illustrating thepotential distribution of the SiC semiconductor device 2 shown in FIG.10. FIG. 11 shows an example where the y-axis thickness Dp from theupper end of the n⁺ type drain region 16 to the upper end of the p⁺ typecollector region 31 is 10 μm.

As shown in FIG. 11, since the upper end of the p⁺ type collector region31 is positioned closer to the surface of the SiC semiconductor layer 10than the upper end of the n⁺ type drain region 16, a relatively highequipotential surface expanding from the n⁺ type drain region 16 can beprevented from reaching the upper end of the p⁺ type collector region31.

A relatively low equipotential surface is distributed over the upper endof the p⁺ type collector region 31. More specifically, an equipotentialsurface of 1.5 through 2 V is distribute over the upper end of the p⁺type collector region 31. As such, it can be seen that the potentialdifference between the p⁺ type collector region 17 and the equipotentialsurface covering the p⁺ type collector region 17 is greater withreference to the structure previously shown in FIG. 4.

FIG. 12 shows the results of researching the relationship between thethreshold voltage of pn junction and the characteristic on-resistancewhile changing the y-axis thickness Dp from the upper end of the n⁺ typedrain region 16 to the upper end of the p⁺ type collector region 31.FIG. 12 is a graph illustrating the threshold voltage of pn junction vs.characteristic on-resistance of a SiC semiconductor device 2 shown inFIG. 10.

The graph illustrated in FIG. 12 shows the results in the case where they-axis thickness Dp from the upper end of the n⁺ type drain region 16 tothe upper end of the p⁺ type collector region 31 is sequentially changedlike 0 μm, 2 μm, 4 μm, 6 μm, 8 μm, and 10 μm.

As shown in the graph of FIG. 12, it was found that lower limit of thecharacteristic on-resistance decreased with an increase in the y-axisthickness Dp in regard to the SiC-MOSFET while the lower limit of thethreshold voltage of pn junction stayed the same in regard to theSiC-IGBT.

As described above, according to the SiC semiconductor device 2, sincethe upper end of the p⁺ type collector region 31 is positioned closer tothe surface of the SiC semiconductor layer than the upper end of the n⁺type drain region 16, the equipotential surface expanding from the n⁺type drain region 16 can be prevented from reaching the upper end of thep⁺ type collector region 31. Thereby, a relatively high equipotentialsurface is prevented from being distributed over the p⁺ type collectorregion 31 while a relatively low equipotential surface can bedistributed over the p⁺ type collector region 31. In this case, thepotential difference between the p⁺ type collector region 31 and theequipotential surface can be made to approach the threshold voltage ofpn junction, the pn junction portion (parasitic diode D) can be turnedon with a relatively small increase in drain voltage.

Further, by increasing the thickness from the upper end of the n⁺ typedrain region 16 to the upper end of the p⁺ type collector region 31, thecharacteristics of on-resistance can be improved compared to a casewhere the n⁺ type drain region 16 and the p⁺ type collector region 31are made to have the same thickness.

Here, the n⁻ type drift region 19 functions as a breakdown voltageholding layer for maintaining the withstand voltage of device (that is,the withstand voltage of the SiC semiconductor device 2). As such, ifthe y-axis thickness Dp is made thicker, the y-axis thickness Td of then⁻ type drift region 19 decreases, and thus the inherent withstandvoltage of device possibly fails to work suitably. Here, the inventorsconceived of a SiC semiconductor 3 shown in FIG. 13.

FIG. 13 is a schematic cross-sectional view of a SiC semiconductordevice 3 according to an embodiment of the present invention.

The SiC semiconductor device 3 is different from the previouslydescribed SiC semiconductor device 1 in that a p⁺ type collector region32 is formed in place of the p⁺ type collector region 17, and that aninsulating film 33 is formed in the rear surface portion of the SiCsemiconductor layer 10 as an example of the insulating layer accordingto the present invention. Other components are the same as those shownin the SiC semiconductor device 1. In FIG. 13, the same referencenumerals are applied to the components corresponding to each componentshown in FIG. 1 previously described and the descriptions for suchcomponents will be omitted.

As shown in FIG. 13, p⁺ type collector regions 32 and n⁺ type drainregions 16 are alternately formed spaced apart from each other in therear surface portion of the SiC semiconductor layer 10.

The p⁺ type collector region 32 is formed right below each unit cell 11(p-type body region 12), and faces the unit cell 11 across the n⁻ typedrift region 19. The x-axis width Wc of the p⁺ type collector region 32according to this embodiment is, for example 10 μm.

FIG. 13 shows an example where the x-axis width Wc of the p⁺ typecollector region 32 is made narrower than the width of the p-type bodyregion 12. The x-axis width Wc of the p⁺ type collector region 32 (theratio of the area of the p⁺ type collector region 32 to the area of then⁺ type drain region 16) can be changed in accordance with the y-axisthickness Td of the n⁻ type drift region 19 as necessary. Therefore,when the y-axis thickness Td of the n⁻ type drift region 19 is madethicker, the p⁺ type collector region 32 may be made wider than thewidth of the p-type body region 12.

Meanwhile, each n⁺ type drain region 16 is formed right below a regionbetween mutually adjacent unit cells 11 (p-type body region 12). Thex-axis width Wd of the n⁺ type drain region 16 according to thisembodiment is, for example, 10 μm. Further, the upper end of the n⁺ typedrain region 16 is positioned at the same depth as the upper end of thep⁺ type collector region 32. The FS region 18 is formed to cover eachupper end of the n⁺ type drain region 16 and the p⁺ type collectorregion 32.

The insulating film 33 is formed in the y-axis direction along thethickness direction of the SiC semiconductor layer 10, and is embeddedin an isolation trench 34 between the n⁺ type drain region 16 and the p⁺type collector region 32. The isolation trench 34 is formed by diggingdown into the semiconductor layer 10 from the rear surface toward thefront surface thereof in the y-axis direction. The isolation trench 34is formed deeper than the n⁺ type drain region 16 and the p⁺ typecollector region 32. A y-axis depth Dt between the upper end of the n⁺type drain region 16 and the upper end of the isolation trench 34 is,for example, 0 μm through 15 μm (0 μm<y-axis depth Dt≦15 μm, 5 μm inthis embodiment). The isolation trench 34 is formed narrower than thex-axis width Wc of the p⁺ type collector region 32 and the x-axis widthWd of the n⁺ type drain region 16.

The insulating film 33 preferably includes an insulating material havingthe lower dielectric constant than SiC, and the insulating material canbe, for example, SiO₂. The insulating film 33 is formed to have thethickness that is the same in size as the depth of the isolation trench34.

Such an insulating film 33 can be made in the following manner: afterforming the n⁺ type drain region 16 and the p⁺ type collector region 32using the method shown in the previously described embodiment, a hardmask selectively having openings corresponding to the regions where theinsulating films 33 are formed is formed on the rear surface of the SiCsemiconductor layer 10.

Next, the isolation trench 34 is formed by digging down into the SiCsemiconductor layer 10 from the rear surface toward the front surface byetching through the hard mask. Next, the insulating film 33 is formed tocover the rear surface of the SiC semiconductor layer 10 by refillingthe isolation trench 34, for example, by CVD method. Thereafter, theunnecessary portions of the insulating film 33 are removed by etch-back.In this way, the insulating film 33 embedded in the isolation trench isformed. A high-resistance layer may be adopted in place of such aninsulating film 33.

In relation to a semiconductor layer using SiC, it is known that anoriginally-nonexistent level exists in a band gap between a conductionband and a valence band due to the existence of point defects (latticedefects) with a given density. Generally, such a level is referred to asa “deep level.” The deep level functions as a carrier trapping center(carrier trap), and thus the resistance of a region having point defects(lattice defects) with a relatively high density becomes high.

The high-resistance layer is a region where the resistance becomeshigher due to the introduction of such a deep level. The point defects(lattice defects) with a given density are formed in the high-resistancelayer by injecting such ions as described later, to thereby form thedeep level. The deep level in the high-resistance layer is closer to amidgap level (that is, an intermediate energy between the minimum energyof the conduction band and the maximum energy of the valence band) thana level formed by a dopant in the n⁻ type drift region 19. The densityof the deep level in the high-resistance layer (the density of pointdefects) is preferably the same (nearly the same) as the density of theimpurities (donor) in the n⁻ type drift region 19 or greater.

Such a high-resistance layer can be made in the following manner: afterforming the n⁺ type drain region 16 and the p⁺ type collector region 32using the method shown in the previously described embodiment, a maskselectively having openings corresponding to the regions where thehigh-resistance layers are formed is formed on the rear surface of theSiC semiconductor layer 10. Then, ion irradiation or electron beamirradiation is carried out.

In the case of ion irradiation, ions of light elements are injected inthe SiC semiconductor layer 10 through a mask. Hydrogen ions (proton),helium ions, boron ions and so forth can listed as the ions of lightelements. When the ions of light elements are adopted, ions can beinjected in positions further deeper than the n⁺ type drain region 16and so forth for the SiC semiconductor layer 10 having higher densitythan a Si semiconductor layer. Thereby, a high-resistance layer with aprescribed y-axis depth Dt (0 μm<y-axis depth Dt≦15 μm, 5 μm in thisembodiment) can be easily formed.

On the other hand, when a high-resistance layer is formed by carryingout electron beam irradiation, the SiC semiconductor layer 10 isirradiated with an electron beam through a mask. Although the conditionsof electron beam irradiation are different depending on the depth of thehigh-resistance layer to be formed, for example, the irradiation energymay be 100 keV through 600 keV, and the amount of electron beamirradiation may be 1×10¹⁵ cm² through 1×10¹⁸ cm². The electron beamirradiation may be single-stage irradiation carrying out onetimeirradiation, or multi-stage irradiation carrying out a plurality ofirradiations.

If the high-resistance layer can be formed at the y-axis depth Dt,p-type impurities (boron, aluminum and so forth) or n-type impurities(phosphorus, arsenic and so forth) may be injected into the SiCsemiconductor layer 10 in place of ion irradiation or electron beamirradiation. In this case, annealing is carried out to the extent thatthe impurities are not activated (for example, the activation rate isless than 1%). The region where the impurities are injected in the SiCsemiconductor layer 10 becomes a high-resistance SiC provided that theimpurities are not activated.

FIG. 14 shows the results of researching the electrical potential of theSiC semiconductor device 3 in the same manner as previously described inFIGS. 4 through 6. FIG. 14 is a view illustrating the distribution ofelectrical potential of a semiconductor device 3 shown in FIG. 13.

As shown in FIG. 14, it was found that a relatively high equipotentialsurface concentrically expanding from the n⁺ type drain region 16 couldbe blocked by the insulating film 33 (high-resistance layer) with theinsulating film 33 (high-resistance layer) formed between the n⁺ typedrain region 16 and the p⁺ type collector region 32.

Further, it can be seen that a relatively low equipotential surface isdistributed over the upper end of the p⁺ type collector region 32,because the equipotential surface expanding from the n⁺ type drainregion 16 can be blocked by the insulating film 33 (high-resistancelayer). More specifically, an equipotential surface of 1.5V through 2Vis distributed over the p⁺ type collector region 32. As such, it can beseen that the potential difference between the p⁺ type collector region17 and the equipotential surface covering the p⁺ type collector region17 is greater with reference to the structure previously shown in FIG.4. From the results of this simulation, it was found that the sameelectrical characteristics as the SiC semiconductor device 2 could beachieved according to the SiC semiconductor device 3.

As described above, according to the SiC semiconductor device 3,insulating film 33 (high-resistance layer) is formed between the n⁺ typedrain region 16 and the p⁺ type collector region 32, and thus arelatively high equipotential surface expanding from the n⁺ type drainregion 16 can be blocked by the insulating film 33 (high-resistancelayer). Thereby, a relatively high equipotential surface can besuppressed from being distributed over the p⁺ type collector region 32while a relatively low equipotential surface can be distributed over thep⁺ type collector region 32.

With this configuration, the potential difference between the p⁺ typecollector region 32 and the equipotential surface can be made toapproach the threshold voltage of pn junction, and thus the pn junctionportion (parasitic diode D) can be turned on with a relatively smallincrease in drain voltage. In this way, the transfer from a smallcurrent region to a large current region can be carried out, and therebythe tradeoff relationship between the characteristics of a small currentregion and the characteristics of a large current region can beimproved. As a result, the SiC semiconductor device 3 excellent in boththe characteristics of a small current region and the characteristics ofa large current region can be provided.

Further, different from the above-described SiC semiconductor device 2,the y-axis thickness Td in the n⁻ type drift region 19 between the p⁺type collector region 32 and the p-type body region 12 is prevented frombeing thinner than the thickness of the n⁻ type drift region 19 betweenn⁺ type drain region 16 and the p-type body region 12 in the SiCsemiconductor device 3, and thus the decrease in withstand voltage of adevice can be effectively suppressed.

FIG. 15 is a schematic cross-sectional view of a SiC semiconductordevice 4 according to an embodiment of the present invention.

The SiC semiconductor device 4 is different from the previouslydescribed SiC semiconductor device 1 in that a p⁺ type contact region 14is not formed, and that a p-type column region 35 is formed below thep-type body region 12. Other components are the same as those shown inthe SiC semiconductor device 1. In FIG. 15, the same reference numeralsare applied to the components corresponding to each component shown inFIG. 1 previously described and the descriptions for such componentswill be omitted.

The p-type column region 35 is integrally formed with and extends fromthe p-type body region 12 in the inner portion from the p-type bodyregion 12. More specifically, the p-type column region 35 is formed toextend from the bottom portion of the p-type body region 12 toward then⁻ type drift region 19 in the y-axis direction along the thicknessdirection of the SiC semiconductor layer 10, to thereby forma pnjunction with the n⁻ type drift region 19. The bottom portion of thep-type column region 35 is positioned between the p-type body region 12and the FS region 18.

As described above, according to the SiC semiconductor device 4, a superjunction (SJ) structure is formed in addition to a Hybrid-MOS structure.The SJ structure allows a depletion layer to expand in the directionalong the interface between the p-type column region 35 and the n⁻ typedrift region 19 (that is in the thickness direction of the n⁻ type driftregion 19) throughout the interface. As a result, a local electric fieldconcentration can be prevented in the n⁻ type drift region 19, and thusan on-resistance value can be decreased while improving a withstandvoltage.

According to such effects of reducing an on-resistance and improving awithstand voltage, the characteristics of threshold voltage of pnjunction vs. characteristic on-resistance shown in previously describedFIG. 3 can be further improved. Further, since the characteristics canbe improved, the impurity concentration of the n⁻ type drift region 19can be decreased. Also, the y-axis thickness Td of the n⁻ type driftregion 19 can be made thinner. Therefore, the SiC semiconductor device 4excellent in both the characteristics of a small current region and thecharacteristics of a large current region while increasing the designflexibility, can be provided.

FIG. 16 is a schematic cross-sectional view of a SiC semiconductordevice 5 according to an embodiment of the present invention.

The SiC semiconductor device 5 is different from the previouslydescribed SiC semiconductor device 1 in that a trench-gate structure inwhich a gate electrode 37 is embedded in a gate trench 36 is formed inplace of the gate electrode 21. Other components are the same as thoseshown in the SiC semiconductor device 1. In FIG. 16, the same referencenumerals are applied to the components corresponding to each componentshown in FIG. 1 previously described and the descriptions for suchcomponents will be omitted.

As shown in FIG. 16, a plurality of gate trenches 36 is formed in they-axis direction along the thickness direction of the SiC semiconductorlayer 10 from the front surface toward the rear surface of the SiCsemiconductor layer 10. The bottom portion of each gate trench 36 ispositioned at the middle along the thickness direction of the SiCsemiconductor layer 10 (n⁻ type drift region 19). The edge portionsconnecting the lateral surface and the bottom portion of each gatetrench 36 are formed in a shape curved outwardly therefrom. Each gatetrench 36 is formed into a U-shape in cross-sectional view. Theconcentration of electric filed on the edge portions can be reducedgiven that each gate trench 36 has an edge formed into a curved shape.

Each gate trench 36 has the gate electrode 37 embedded therein across agate insulating film 38. The gate electrode 37 has a surface flush withthe surface of the SiC semiconductor layer 10. The materials of the gateelectrode 37 and the gate insulating film 38 are the same as those inthe previously described embodiment. A p-type body region 40 thatdefines the unit cell 11 is formed between mutually adjacent gatetrenches 36. A p-type region 39 is formed in a region between the bottomportion of each gage trench 36 and the upper end of the FS region 18.

The p-type region 39 is formed along the bottom portion of each gatetrench 36. The p-type region 39 covers the edge portion of each gatetrench 36, and this p-type region 39 serves to alleviate the electricfield concentration at the edge portion of each gate trench 36. Thep-type region 39 may be formed spaced apart from the bottom portion ofeach gate trench 36.

The bottom portion of the p-type body region 40 is positioned betweenthe surface of the SiC semiconductor layer 10 and the bottom portion ofthe gate trench 36 in the y-axis direction along the thickness directionof the SiC semiconductor layer 10. The end of the p-type body region 40forms a part of the gate trench 36 in the x-axis direction along thesurface of the SiC semiconductor layer 10. That is, the p-type bodyregion 40 is electrically connected to the gate electrode 37 with thegate insulating film 38 interposed therebetween. In this embodiment, then⁻ type drift region 19 is the region between the p-type body region 40and the FS region 18. An n-type source region 41 is formed in the innerportion of the p-type body region 40.

The n-type source region 41 is formed shallower than the p-type bodyregion 40 in the y-axis direction along the thickness direction of theSiC semiconductor layer 10. The end of the n-type source region 41 formsa part of the gate trench 36 in the x-axis direction along the surfaceof the SiC semiconductor layer 10. That is, the n-type source region 41is electrically connected to the gate electrode 37 with the gateinsulating film 38 interposed therebetween. A p-type channel region 42is a region along the gate trench 36 between the lower end of the n-typesource region 41 and the lower end of the p-type body region 40 in they-axis direction along the thickness direction of the SiC semiconductorlayer 10. A p⁺ type contact region 43 is formed so as to pass throughthe n-type source region 41.

The p⁺ type contact region 43 passes through the n-type source region41, and is formed so as to cross the boundary between the n-type sourceregion 41 and the p-type body region 40. The p⁺ type contact region 43has an impurity concentration higher than the p-type body region 40.

An insulating film 44 covering the gate electrode 37 is formed on theSiC semiconductor layer 10. A contact hole 45, which selectively exposea part of the n-type source region 41 and the p⁺ type contact region 43,is formed in the insulating film 44. The source electrode 24 iselectrically connected to the p type body region 40, a part of the ntype source region 41, and the p⁺ type contact region 43 in the contacthole 45.

As described above, according to the SiC semiconductor device 5, atrench-gate structure is formed in addition to the Hybrid-MOS structure.Even with this configuration, the same effects as the effects in thepreviously described embodiment can be produced.

FIG. 17 is a schematic cross-sectional view of a SiC semiconductordevice 6 according to an embodiment of the present invention.

The SiC semiconductor device 6 is different from the previouslydescribed SiC semiconductor device 5 in that a double-trench structureincluding a source trench 46 is formed in addition to the gate trench36, that p-type region 39 is not formed at the bottom portion of thegate trench 36, and that a p-type body region 47, an n-type sourceregion 48, and a p⁺ type contact region 50 are formed in place of thep-type body region 40, the n-type source region 41, and the p⁺ typecontact region 43. Other components are the same as those shown in theSiC semiconductor device 5. In FIG. 17, the same reference numerals areapplied to the components corresponding to each component shown in FIG.16 previously described and the descriptions for such components will beomitted.

As shown in FIG. 17, the source trench 46 is formed at the center ofeach unit cell 11. A plurality of the source trenches 46 is formedextending from the front surface toward the rear surface of the SiCsemiconductor layer 10 in the y-axis direction along the thicknessdirection of the SiC semiconductor layer 10. The source trench 46 isformed at the same depth as the depth of the gate trench 36. The edgeportions connecting the lateral portion and the bottom portion of thesource trench 46 are formed in a shape curved outwardly from the sourcetrench 46. The source trench 46 is formed into a U-shape incross-sectional view. The concentration of electric filed on the edgeportions can be alleviated given that the source trench 46 has an edgeformed into a curved shape.

The p-type body region 47 is formed along the surface of the SiCsemiconductor layer 10, and the lateral portion and the bottom portionof the source trench 46. The p-type body region 47 formed along thelateral portion and the bottom portion of the source trench 46 forms apart of the lateral portion and the bottom portion of the source trench46. Although FIG. 17 illustrates an example where the p-type body region47 formed along the lateral portion of the source trench 46 is formedthinner than the p-type body region 47 formed along the bottom portionof the source trench 46, the p-type body region 47 may be formed to havethe same thickness.

The n-type source region 48 is formed between the gate trench 36 and thesource trench 46 in the surface portion of the SiC semiconductor layer10. The ends of the n-type source region 48 form a part of the gatetrench 36 and a part of the source trench 46 in the x-axis directionalong the surface of the SiC semiconductor layer 10. The n-type sourceregion 48 is formed shallower than the p-type body region 47 in they-axis direction along the thickness direction of the SiC semiconductorlayer 10. A p-type channel region 49 is a region between the lower endof the n-type source region 48 and the lower end of the p-type bodyregion 47 along the gate trench 36 in the y-axis direction.

The p⁺ type contact region 50 formed at the bottom portion of the sourcetrench 46. That is, the p⁺ type contact region 50 forms a part of thebottom portion of the source trench 46. The bottom portion of the p⁺type contact region 50 is positioned between the bottom portion of thesource trench 46 and the bottom portion of the p-type body region 47formed along the bottom portion of the source trench 46.

The source electrode 24 gets into the contact hole 45 from the surfaceof the insulating film 44 and gets into the source trench 46 from thecontact hole 45. The source electrode 24 is electrically connected tothe p-type body region 47, the n-type source region 48, and the p⁺ typecontact region 50 in the contact hole 45 and the source trench 46.

As described above, according to the SiC semiconductor device 6, adouble trench structure including the gate trench 36 and the sourcetrench 46 is formed in addition to the Hybrid-MOS structure. Even withthis configuration, the same effects as the effects in the previouslydescribed embodiment can be produced.

Although this embodiment illustrates an example where the p-type region39 is not formed at the bottom portion of the gate trench 36, the p-typeregion 39 may be formed at the bottom portion of each gate trench 36 asin the previously described SiC semiconductor device 5. In this case,the p-type region 39 and the p⁺ type contact region 50 may be formed tohave the same concentration at the same depth. With this configuration,the p-type region 39 and the p⁺ type contact region 50 may be made inthe same manufacturing step.

Planar Shape of p⁺ Type Collector Region

The p⁺ type collector regions 17, 31, 32 shown in the previouslydescribed SiC semiconductor devices 1 through 6 may have planar shapesas shown in FIGS. 18 through 20. FIGS. 18 through 20 are plan viewsillustrating the planar shapes of the p⁺ type collector regions 17, 31,32 according to the previously described embodiments. The planar shapeimplies the shape of the p⁺ type collector regions 17, 31, 32 in planview when the SiC semiconductor layer 10 is viewed along the normaldirection.

As shown in FIG. 18, the p⁺ type collector regions 17, 31, 32 maybeformed in a rectangular shape (striped shape). FIG. 18 shows an exampleof the rectangular-shaped p⁺ type collector regions 17, 31, 32. In thiscase, the x-axis width Wc is defined as the width in the transversaldirection.

Further, as shown in FIG. 19, the p⁺ type collector regions 17, 31, 32may be formed in a polygonal shape. FIG. 19 shows an example of thehexagonal-shaped p⁺ type collector regions 17, 31, 32. In this case, thex-axis width Wc is defined as the width of a vertical line connectingtwo sides. If a vertical line connecting two sides cannot be drawn as ina pentagonal shape, the x-axis width Wc of the p⁺ type collector regions17, 31, 32 may be defined by the width of the diagonal line.

Further, as shown in FIG. 20, the p⁺ type collector regions 17, 31, 32may be formed in a circular shape. In this case, the x-axis width Wc ofthe p⁺ type collector regions 17, 31, 32 may be defined as a diameter ofthe circle. Of course, the p⁺ type collector regions 17, 31, 32 may beformed in an elliptical shape. In the case of an elliptical shape, thex-axis width Wc of the p⁺ type collector regions 17, 31, 32 is definedas the width of the transversal axis.

According to the previously described SiC semiconductor devices 1through 6, such p⁺ type collector regions 17, 31, 32 are selectivelyformed in the rear surface portion of the SiC semiconductor layer 10.

Layout Examples of p⁺ Type Collector Regions and n⁺ Type Drain Regions

Further, the p⁺ type collector regions 17, 31, 32 and the n⁺ type drainregions 16 may be laid out as shown in FIGS. 21 and 22. FIGS. 21 and 22are plan views illustrating layout examples 51, 52 of the p⁺ typecollector regions 17, 31, 32 and the n⁺ type drain regions 16.

As shown in the layout example 51 of FIG. 21, a plurality of the p⁺ typecollector regions 17, 31, 32 is arrayed in stripes spaced apart fromeach other. The n⁺ type drain regions 16 are arrayed in stripes betweenthe mutually adjacent p⁺ type collector regions 17, 31, 32. The p⁺ typecollector regions 17, 31, 32 selectively include a region formedrelatively wide and a region formed relatively narrow compared to therelatively wide region. In this layout example 51, the x-axis width Wcof the p⁺ type collector regions 17, 31, 32 is defined by the width of aregion formed widest from among a plurality of the widths of the p⁺ typecollector regions 17, 31, 32 in a direction orthogonal to the stripedirection.

According to the layout example 52 of FIG. 22, a plurality of the p⁺type collector regions 17, 31, 32 rectangular shaped in plan view isarranged in a matrix array. Outwardly from the rectangular-shaped p⁺type collector regions 17, 31, 32, an annular rectangular n⁺ type drainregions 16, an annular rectangular p⁺ type collector region 17, 31, 32,and another annular rectangular n⁺ type drain regions 16 are arrayed inthis order with the rectangular-shaped p⁺ type collector regions 17, 31,32, as the center. A lattice-shaped p⁺ type collector region 17, 31, 32is arrayed so as to mark out the n⁺ type drain regions 16 arrayedoutermost from the rectangular-shaped p⁺ type collector regions 17, 31,32. In this layout example 52, the x-axis width Wc of the p⁺ typecollector region 17, 31, 32 is defined by the width of therectangular-shaped p⁺ type collector regions 17, 31, 32.

According to the layout examples 51, 52, when a prescribed thresholdvoltage of pn junction is applied to the drain electrode 25 withreference to the source electrode 24, the p⁺ type collector region 17,31, 32 formed relatively wide is first turned on. Then, the p⁺ typecollector regions 17, 31, 32 formed relatively narrow are sequentiallyturned on triggered by the turn-on of the wide-formed p⁺ type collectorregion 17, 31, 32.

As such, even when a threshold voltage of pn junction normally notenough to turn on the narrow-formed p⁺ type collector regions 17, 31, 32is applied thereto, the narrow-formed p⁺ type collector regions 17, 31,32 can be turned on in response to the turn-on of the wide-formed p⁺type collector region 17, 31, 32. As a result, characteristics in thethreshold voltage of pn junction can be improved.

Although some embodiments of the present invention have been described,other embodiments may also be practiced according to the presentinvention.

For example, the structures of the SiC semiconductor devices 1 through 6in each embodiment previously described may be selectively combined.Accordingly, for example, the p⁺ collector region 17 or the insulatingfilm 33 (high-resistance layer) of the SiC semiconductor devices 2, 3may be combined with the SiC semiconductor devices 1, 4-6.

Further, although an example in which the n⁺ drain region 16 is formedright below the region between mutually adjacent unit cells 11 isdescribed in the previously described embodiment, another example inwhich the n⁺ drain region 16 is formed outside the region right belowthe region between mutually adjacent unit cells 11 (for example rightbelow the unit cell 11) may be adopted.

Further, although a previously described embodiment shows an example inwhich the isolation trench 34 is formed to have the lateral portionorthogonal to the rear surface of the SiC semiconductor layer 10, theisolation trench 34 may be formed to have a trapezoidal shape (taperedshape) in cross-sectional view with the opening width getting narrowerfrom the opening toward the bottom portion. Similarly, the isolationtrench 34 may be formed to have a trapezoidal shape (tapered shape) incross-sectional view with the opening width getting wider from theopening toward the bottom portion. Further, the isolation trench 34 maybe formed to have a trapezoidal shape (tapered shape) in cross-sectionalview with the opening width getting wider from the opening toward thebottom portion. Further, the isolation trench 34 maybe formed to beinclined toward the inner portion of the n⁺ type drain regions 16 incross-sectional view. Further, the isolation trench 34 may be formedwider than the x-axis width Wc of the p⁺ type collector region 32 and/orthe x-axis width Wd of the n⁺ drain region 16 in the x-axis directionalong the surface of the SiC semiconductor layer 10.

Further, in the previously described embodiment, the high-resistancelayer may be formed to be inclined toward the inner portion of the n⁺type drain regions 16 in cross-sectional view. Further, thehigh-resistance layer may be formed wider than the x-axis width Wc ofthe p⁺ type collector region 32 and/or the x-axis width Wd of the n⁺drain region 16 in the x-axis direction along the surface of the SiCsemiconductor layer 10.

Further, although a previously described embodiment shows an example inwhich the gate trench 36 and/or the source trench 46 are formed to havethe lateral portion orthogonal to the surface of the SiC semiconductorlayer 10, the gate trench 36 and/or the source trench 46 may be formedto have a trapezoidal shape (tapered shape) in cross-sectional view withthe opening width getting narrower from the opening toward the bottomportion.

Further, a configuration with an inverted conductivity type may beadopted in each previously described embodiment. That is, in eachpreviously described embodiment, a p-type maybe replaced by an n-type oran n-type maybe replaced by a p-type.

The SiC semiconductor devices 1 through 6 according to the presentinvention may be incorporated in a power module used in an invertercircuit that forms a drive circuit for driving an electric motor used asa power source for electric cars (hybrid cars included), trains,industrial robots and so forth. Further, the SiC semiconductor devices 1through 6 may also be incorporated in a power module used in an invertercircuit that converts electric power generated by solar batteries, windpower generators, and other power generating devices (particularlyprivate electric generator) so as to be consistent with electric powerfrom commercial power supply.

It is to be understood that variations and modifications can be madewithout departing from the scope and spirit of the present invention.

This application corresponds to Patent Application No. 2014-085615submitted to Japanese Patent Office on Apr. 17, 2014, and the entirecontents of this application are hereby incorporated by reference.

DESCRIPTION OF THE REFERENCE NUMERALS

-   1 SiC semiconductor device-   2 SiC semiconductor device-   3 SiC semiconductor device-   4 SiC semiconductor device-   5 SiC semiconductor device-   6 SiC semiconductor device-   10 SiC semiconductor device-   11 Unit cell-   12 p-type body region-   13 n-type source region-   16 n⁺ drain region-   17 p⁺ collector region-   18 Field stop (FS) region-   19 n⁻ type drift region-   20 Gate insulating film-   21 Gate electrode-   31 p⁺ collector region-   32 p⁺ collector region-   33 Insulating film-   36 Gate trench-   37 Gate electrode-   38 Gate insulating film-   40 P-type body region-   41 n-type source region-   47 P-type body region-   48 n-type source region-   Td y-axis thickness-   Wc x-axis width-   Wd x-axis width

1. A semiconductor device comprising: a semiconductor layer of SiC of afirst conductivity type; a plurality of body regions of a secondconductivity type formed in the surface portion of the semiconductorlayer with each body region forming a unit cell; a source region of thefirst conductivity type formed in the inner portion of the body region;a gate electrode facing the body region across a gate insulating film; adrain region of the first conductivity type and a collector region ofthe second conductivity type formed in the rear surface portion of thesemiconductor layer, the drain region and the collector region adjoiningeach other; and a drift region between the body region and the drainregion, wherein the collector region is formed such that the collectorregion covers a region including at least two unit cells in the x-axisdirection along the surface of the semiconductor layer.
 2. Asemiconductor device comprising: a semiconductor layer of SiC of a firstconductivity type; a body region of a second conductivity type formed inthe surface portion of the semiconductor layer; a source region of thefirst conductivity type formed in the inner portion of the body region;a gate electrode facing the body region across a gate insulating film; adrain region of the first conductivity type and a collector region ofthe second conductivity type formed in the rear surface portion of thesemiconductor layer, the drain region and the collector region adjoiningeach other; and a drift region between the body region and the drainregion, wherein an x-axis width Wc of the collector region along thesurface of the semiconductor layer is made at least two times greaterthan a y-axis thickness Td of the drift region along the thicknessdirection of the semiconductor layer.
 3. A semiconductor deviceaccording to claim 1, wherein the drain region has an x-axis widthgreater than or equal to that of the collector region.
 4. Asemiconductor device comprising: a semiconductor layer of SiC of a firstconductivity type; a body region of a second conductivity type formed inthe surface portion of the semiconductor layer; a source region of thefirst conductivity type formed in the inner portion of the body region;a gate electrode facing the body region across a gate insulating film; adrain region of the first conductivity type and a collector region ofthe second conductivity type formed in the rear surface portion of thesemiconductor layer, the drain region and the collector region adjoiningeach other; a drift region between the body region and the drain region;and an insulating layer disposed between the drain region and thecollector region, the insulating layer being formed deeper than thedrain region and the collector region from the rear surface of thesemiconductor layer in the y-axis direction along the thicknessdirection of the semiconductor layer.
 5. A semiconductor deviceaccording to claim 4, wherein the insulating layer includes aninsulating film or a high-resistance layer.
 6. A semiconductor deviceaccording to claim 4, wherein the insulating layer includes aninsulating material having a dielectric constant lower than SiC.
 7. Asemiconductor device according to claim 6, wherein the insulating layerincludes SiO₂.
 8. A semiconductor device comprising: a semiconductorlayer of SiC of a first conductivity type; a body region of a secondconductivity type formed in the surface portion of the semiconductorlayer; a source region of the first conductivity type formed in theinner portion of the body region; a gate electrode facing the bodyregion across a gate insulating film; a drain region of the firstconductivity type and a collector region of the second conductivity typeformed in the rear surface portion of the semiconductor layer, the drainregion and the collector region adjoining each other; and a drift regionbetween the body region and the drain region; wherein the upper end ofthe collector region is positioned closer to the surface of thesemiconductor layer than the upper end of the drain region in the y-axisdirection along the thickness direction of the semiconductor layer.
 9. Asemiconductor device according to claim 8, wherein the rear surface of asemiconductor layer extends across the interface between the drainregion and the collector region to make both surfaces flush with eachother.
 10. A semiconductor device according to claim 1, furthercomprising: a field stop region formed to extend over the drain regionand the collector region in the x-axis direction along the surface ofthe semiconductor layer, the field stop region being disposed betweenthe drift region, and the drain region and the collector region.
 11. Asemiconductor device according to claim 1, wherein the semiconductordevice includes a planar-gate structure where the gate electrode isdisposed on the semiconductor layer.
 12. A semiconductor deviceaccording to claim 1, wherein the semiconductor device includes atrench-gate structure where the gate electrode is embedded in a trenchformed in the semiconductor layer.